TERM CONSULTANCY FOR AIR VENTILATION ASSESSMENT SERVICES Cat. A Term Consultancy for Expert Evaluation and Advisory Services on Air Ventilation Assessment (PLNQ 37/2007) TERM CONSULTANCY FOR AIR VENTILATION ASSESSMENT SERVICES Cat. A – Term Consultancy for Expert Evaluation and Advisory Services on Air Ventilation Assessment (PLNQ 37/2007) Final Report - Mid Levels West March 2008 ………………………………………. by Professor Edward Ng Department of Architecture, CUHK, Shatin, NT, Hong Kong T: 26096515 F:26035267 E: [email protected] W: www.edwardng.com Final Report Page 1 of 48 14 March 2008 TERM CONSULTANCY FOR AIR VENTILATION ASSESSMENT SERVICES Cat. A Term Consultancy for Expert Evaluation and Advisory Services on Air Ventilation Assessment (PLNQ 37/2007) The Study Area Final Report Page 2 of 48 14 March 2008 TERM CONSULTANCY FOR AIR VENTILATION ASSESSMENT SERVICES Cat. A Term Consultancy for Expert Evaluation and Advisory Services on Air Ventilation Assessment (PLNQ 37/2007) Expert Evaluation Report of Mid-Levels West Executive summary 0.1 Wind Availability: (a) The prevailing summer wind of the study area comes from the south. The prevailing annual wind of the study area comes from the East and North-East. (c) The hills south of the study area shield and reduce the air ventilation potentials. They also complicate the wind regime of the study areas when the summer wind comes from the south. (b) At 500m above ground, it is estimated that the site wind is on average in the order of 6 to 8 m/s; and at the top of the urban canopy layer (UCL) (about 100 to 150m), it is estimated that the site wind is on average in the order of 3-5 m/s. (e) The north facing hill slopes is a useful local cool air production source that can benefit the study area, this source of cool air (can be 2-3 degree C cooler) can create downhill (katabatic) air ventilation in the order of 0.5 to 1.0 m/s. 0.2 Topography and Valleys: (a) The valleys on both sides of the Peak are channels of air movement to and across the study area, especially when the important summer wind comes from the south. (i) One of the channels is over the air space of Hong Kong University (HKU), and extends north towards Sai Ying Pun. (ii) The other is from Tregunter Path, and extends north towards the Central District. They should be respected. 0.3 Vegetation and Greeneries (a) Currently, there are a number of noticeable Vegetated Air Paths across the study area. They are useful and should be respected. (i) from Kotewall Road south to West End Park, through Bonham Road Government Primary School to King George V Memorial Park just north of the study area (partially obstructed); (ii) from the middle of Conduit Road, through Caine Road Garden on Seymour Road to Blake Garden just outside of the study area (partially obstructed); (iii) the Peak Tramway; and (iv) along Borrett Road. 0.4 Existing conditions: (a) The areas north of Conduit Road between HKU and Hong Kong Zoological and Botanical Gardens (HKZBG) are denser with higher towers and large podiums. Air ventilation is reduced. Careful interventions with air paths and open spaces are desirable to improve the wind environment. Currently, when wind comes from the East and North-East, the streets parallel to the wind are useful air paths; wind velocity ratio (VR) of around 0.1 can be expected. When wind comes from the Final Report Page 3 of 48 14 March 2008 TERM CONSULTANCY FOR AIR VENTILATION ASSESSMENT SERVICES Cat. A Term Consultancy for Expert Evaluation and Advisory Services on Air Ventilation Assessment (PLNQ 37/2007) south, most of the streets are perpendicular to the wind, the tall buildings form deep canyons; VR of around 0.05 can be expected. That is to say, only about 5% of the available wind of the site at the top of the atmospheric boundary layer can be enjoyed by the pedestrian at 2m above ground – this is estimated to be on average in the order of 0.5m/s; hence, it is very important to find ways to improve the air ventilation performance of the area. (b) Given that the buildings of the study area in (a) are already tall which result in high building height to street width ratios (H/W), adjusting the building heights a little bit one way or another would not make a difference to air ventilation. For example, all else being equal, a deep street canyon of H/W of 4:1 or 5:1 would have very similar air ventilation performance at pedestrian level (2m above ground). In such a circumstance, the most effective way to improve air ventilation is to introduce air paths through building gaps. (c) Most of the other areas should not have major air ventilation issues due to their higher altitude, proximity to green areas, unobstructed aspects, and lower density. Their air ventilation implication to the neighbours will be discussed later. 0.5 The Study Area’s Strategic Importance: (a) The study area is strategic in that it lies on the south of the main urban fabric of Hong Kong Island from Sai Ying Pun to Admiralty. The planning of the study area must take into account its air ventilation contribution to this urban fabric. That is to say, the study area cannot be planned in isolation without a consideration of its neighbours. When the summer wind comes from the south, currently, the tall and dense buildings in the study area make it very difficult for the wind moving into the urban fabric north of the study area. (b) To make a useful contribution to the air ventilation of the urban fabric to the north of the study area, it is very useful to have proper air paths (both by preserving existing air paths and by creating new ones); buildings of the study area must not be too high or too bulky adding roughness and blockage to the incoming winds; and given that some parts of the study area [refer to 0.4(a)] are already “over-developed”, adding further building volumes is not recommended and is in general not desirable for air ventilation. (c) As a principle, to maximize the neighbour’s air ventilation, buildings of the study area should be as low as possible. However, given the same building volume, lower buildings occupy more of the site, have higher site coverage, and thus reduce air ventilation within the study area. To optimize and balance the conditions, once the heights are kept as low as possible, the lower and “fatter” buildings of the study area need to be carefully designed and positioned to maintain some air paths through their north-south direction. 0.6 The Study Area with Potential Re-Developments: (a) Based on the information provided by Planning Department, a lot of sites in the study areas have development potentials. They are yet developed to their maximum permitted intensity and can be re-developed with taller, larger and bulkier Final Report Page 4 of 48 14 March 2008 TERM CONSULTANCY FOR AIR VENTILATION ASSESSMENT SERVICES Cat. A Term Consultancy for Expert Evaluation and Advisory Services on Air Ventilation Assessment (PLNQ 37/2007) buildings, and with big podiums. This will further reduce air ventilation, and is not desirable. Adding further building volumes is not recommended. Due to considerations other than air ventilation, should they be allowed, proper planning and design control are extremely important. For example, introducing air paths and building permeability are useful and beneficial strategies. (b) To illustrate, consider a “worst case scenario” that three to four long lines of tall buildings of 30-40 storeys (or higher) with large podiums form a continuous wall like barrier between the hills and the urban fabric from Sai Ying Pun to Admiralty. Due to the layout of existing street patterns, these lines of tall buildings will be perpendicular to the critical summer wind from the south. This scenario should be avoided. (c) If “limited” re-development could be considered, a number of strategies should be borne in mind. (i) The GIC, O and GB zones in the study area should be respected. They provide useful air spaces in the study area. They should not be further developed with tall buildings or re-zoned for bulkier development. (ii) North-south orientated air paths should be provided by designating non- building areas. It is recommended that air paths be incorporated for each development. (iii) The scale and size of the podium should be restricted. Permeability should be designed into the buildings. Properly perforated podiums provide useful air paths for air ventilation. (iv) Towers must not be designed to occupy the full frontage of the site, especially the east-west direction. Appropriate gaps between the towers, and within the towers, must be left to allow air ventilation through them. (v) Greeneries and landscaping should be encouraged and enhanced. Existing trees must not be disturbed. 0.7 The Study Area with Height Restrictions: (a) First of all, the consultant opines that, given the congested building environment with lots of tall buildings, and the possibility of having many more taller and bulkier buildings within the study area, height restrictions may not be the most important consideration for air ventilation. The air ventilation in some denser parts of the study area is already poor, and will worsen further due to the potential re- development. Given the scenario, height restrictions on its own is NOT going to help air ventilation. (b) As a principle, for air ventilation, a variation of building heights in close proximity is preferred.